The present invention relates in general to a sample changer for transferring radioactive samples between a hot cell and a measuring apparatus. It relates in particular to such a sample changer for use with a hybrid K-edge densitometer. It also relates to a hybrid K-edge densitometer measuring facility.
A hybrid K-edge densitometer (HKED) measuring facility, which allows to determine uranium and plutonium concentrations in liquors, as obtained e.g. after dissolution of various types of nuclear materials (e.g. irradiated reactor fuel elements), is disclosed in the publication: xe2x80x9cThe Hybrid K-Edge/K-XRF Densitometer: Principlesxe2x80x94Designxe2x80x94Performancexe2x80x9d, H. Ottmar, H. Eberle, Report KfK 4590, February 1991, Kernforschungszentrum Karlsruhe.
This prior art HKED measuring facility comprises a hot cell, for safely handling the samples to be measured with telemanipulators, the HKED measuring apparatus itself, which is located outside the hot cell, and a sample transfer tube, which is connecting the measuring apparatus to the hot cell. This sample transfer tube consists of a stainless steel tube with an outer diameter of 8 cm, which extends from the measuring apparatus through an existing adapter flange into the hot cell. It includes a charging/discharging port in the hot cell and a measuring window section traversed by the X-ray measuring beams in the measuring apparatus. A transfer channel extends axially through the stainless steel tube from the charging/discharging port into the measuring window section. It has a rectangular cross-section to accommodate a monobloc sledge, so that the latter is easily gliding through the transfer channel. This monobloc sledge has a single compartment for receiving therein a receptacle containing two vials with the sample to be measured. The rear end section of the stainless steel tube is closed and includes a micro-switch and magnet.
For carrying out a measurement in the prior art HKED facility, the receptacle with the sample is placed into the sledge when the latter is located in the charging/discharging port of the sample transfer tube. The sledge is then transferred through the transfer channel of the sample transfer tube from the charging/discharging port into the measuring window section. To carry out this transfer, the operator has to manipulate, with the telemanipulators of the hot cell, a rod of about 80 cm, in order to push the sledge through the transfer channel into its measuring position within the measuring window section. When the sledge is positioned in its measuring position, it actuates the micro-switch in the rear end section of the transfer tube, thus enabling the measurement procedure. The magnet in the rear end section of the transfer tube maintains the sledge in place during the measurement, thus assuring a reproducible positioning of the sample in the measuring beams. When the measurement is finished, the operator uses again the telemanipulators and the rod to pull the sledge back into the charging/discharging port, where the receptacle with the samples is lifted out of the sledge.
The sample transfer tube of the prior art HKED facility has following indisputable advantages:
it provides a safe containment for the transfer of the samples between the hot cell and the measuring apparatus;
it provides a high degree of operational reliability under the severe environment conditions in the hot cell, which are characterised e.g. by the presence of acid vapours and extreme levels of gamma radiation;
it is very compact, so that it can be mounted in a standard flange adapter of a hot cell;
it allows to position the focal spot of a shielded X-ray tube in the measuring window section at a very short distance from the center of the sample to be measured;
it assures a very accurate reproducible positioning of the sample in the measurement position without relying on electric or electronic equipment located inside the containment.
A major disadvantage of the prior art sample transfer tube is that it requires manual intervention for changing the sample after a measurement.
A technical problem underlying the present invention is to automate transfer and exchange of samples between the hot cell and the measuring apparatus located outside the hot cell, while generally maintaining the above mentioned advantages of the manually operated sample transfer tube. This problem is solved by a sample changer as claimed in claim 1.
The sample changer of the present invention includes a tubular containment having a charging/discharging port, which is introduced into the hot cell; a transfer section, which extends into the measuring apparatus outside the hot cell; a measuring window section, which is traversed by a measuring beam inside the measuring apparatus; and an closed rear end section, which is located at the opposite end of the charging/discharging port. A recipient with at least one compartment for receiving therein a radioactive sample is arranged in a transfer channel extending axially through the tubular containment between the charging/discharging port and the measuring window section. When located in the charging/discharging port within the hot cell, this recipient can be charged and discharged by means of telemanipulators. In accordance with an important aspect of the present invention, a threaded spindle is rotatably housed in a spindle channel arranged in the tubular containment below the transfer channel. This threaded spindle extends between the charging/discharging port and the measuring window section. A stepping motor is located at the outside of the tubular containment and is connected to the threaded spindle via a coupling sealingly passing through the closed rear end section of the tubular containment. A longitudinally guided support carriage, which is supporting the recipient in the transfer channel, engages the threaded spindle so as to be subjected to a translational movement upon rotation of the spindle. It follows that the transfer of the samples from the hot cell into the measuring apparatus and vice versa, can be entirely automated and no longer needs remote handling operations with telemanipulators. The linear drive, which is used in the sample changer of the present invention for automating the transfer of the samples, has a high operational reliability and is capable of providing an excellent positioning accuracy of the samples in the measuring apparatus. It is integrated into the sample changer in such a way that the cross-section of the containment need not be increased with regard to a traditional sample transfer tube with manual sample transfer. Thus it will be possible to install the sample changer into an existing hot cell adapter flange, just as the prior art transfer tube. Last but not least, it will be appreciated in particular that the sample changer of the present invention is characterized by a strict separation of electrical and mechanical components of its drive. Only fail-safe mechanical components are kept within the alpha containment of the hot cell with its hostile ambient conditions (radiation, acid vapors . . . ,). Electrical components, as the stepping motor, are located outside the containment, so that they are easily accessible for maintenance and replacement.
In a preferred embodiment of the invention, the sample changer further includes a plug-in coupling device, for coupling one end of the threaded spindle to the coupling, and a bearing block for supporting the opposite end of the threaded spindle. This bearing block is slidably fitted in the spindle channel, and the spindle channel is axially accessible from the hot cell, so that the threaded spindle and its bearing block can be axially withdrawn from the spindle channel into the hot cell. It follows thatxe2x80x94for maintenance and or replacementxe2x80x94the contaminated mechanical components of the linear drive may be safely withdrawn with the help of telemanipulators into the hot cell.
To be capable of easily dismounting the support carriage by means of telemanipulators, without removing the spindle, the carriage preferably includes threaded engaging means that are engaging exclusively the upper half of the threaded spindle. It follows that the carriage may be simply lifted from the threaded spindle by means of the telemanipulators.
In a preferred embodiment, the support carriage includes two support blocks, wherein each of these support blocks comprises a cylindrically curved threaded surface engaging exclusively the upper half of the threaded spindle. This block rests advantageously by means of downwardly oriented runners on two lateral support surfaces in the spindle channel and is laterally guided in the spindle channel.
The support carriage and the recipient could of course be fixed together so as to form one single element. However, in order to facilitate maintenance and to allow the use of different types of recipients, it is suggested to conceive the recipient and the carriage as two independent elements, wherein interlocking means on the recipient and the support carriage are co-operating for reproducibly positioning the recipient on the support carriage. Thus the recipient can be removed from the support carriage, without affecting the positioning accuracy. In a preferred embodiment, the support carriage includes for example a support plate connecting the support blocks together and providing a support surface for the recipient, wherein the recipient and the support plate include interlocking means co-operating for reproducibly positioning the recipient on the support plate.
In a preferred embodiment of the invention, the recipient is a magazine including several compartments arranged in axial alignment within the magazine. Control means control the stepping motor so as to adjust each of the different compartments of the magazine subsequently in the measuring beam. It will be appreciated that such a sample changer allows fully automated measurements on several measurement samples.
For reproducibly positioning the different compartments of the sample magazine into the measuring beam, the control means include advantageously a slit aperture in the magazine and computer means which are operatively coupled to means for measuring the intensity of the measuring beam passing through the slit aperture and to the stepping motor. The slit aperture is driven through the measuring beam and the computer means compute a final reference position of the stepping motor corresponding to the position of the stepping motor in which the intensity of the measuring beam passing through the slit aperture is maximum. This final reference position is then used for computing the number of steps to be executed by the stepping motor in order to position each of the compartments of the magazine accurately in the measuring beam.
In order to facilitate the positioning operation with the slit aperture, the sample changer further includes a position detector to be triggered by the support carriage, when the latter is in a predetermined position in the measuring window section. Computer means, which are operatively coupled to the position detector and to the stepping motor, compute an initial reference position of the stepping motor corresponding to the position of the stepping motor in which the position detector is triggered by the support carriage. This initial reference position is then used for calculating the number of steps to be executed by the stepping motor in order to position the slit aperture at a predetermined distance from the measuring beam. The position detector is preferably an inductive position detector housed in a leakproof protection sheath within the rear end section of the containment and detecting the presence of a metallic flag fixed to the transfer carriage.
It will be appreciated that the present invention also provides a Hybrid K-Edge Densitometer (HKED) measuring facility for automated measurements on highly radioactive materials.